Lithium ion secondary batteries are currently the best portable energy storage device for the consumer electronics market. Improved safety over conventional, fully liquid electrolytes provides a compelling rationale for use of polymer electrolytes in rechargeable lithium batteries, but these polymers often show insufficient conductivity or poor mechanical properties The dual ion-conducting nature of most polymer electrolytes also poses problems. Investigations of the transport properties indicate that cationic transference numbers are non-unity or even negative, indicating substantial transport by anionic complexes, particularly at high salt concentration. Concentration gradients caused by the mobility of both cations and anions in the electrolyte arise during cell operation, resulting in premature cell failure. This is a more severe problem than in conventional liquid electrolytes because of the lower salt diffusion coefficients and the relative immobility of the polymer hosts.
Attempts to design single ion conductors based on polymer-electrolytes with fixed negative charges on the polymer have met with limited success; conductivities are relatively too low for practical use. Still, the ease of film fabrication, ability to withstand electrode volume changes, and low temperature operation of a well-designed polymer-based system provide distinct advantages over many ceramic single-ion conductors.
The conductivities of lithium-containing polymer-clay nanocomposites are greatly enhanced over synthetic polymer single-ion conductors because only cations are mobile in these materials. Preparation is simpler, they are self-supporting, and generally have excellent mechanical properties.
The modification of polymer properties by the addition of another material, that is, a filler, has been studied for many years. Common fillers such as glass fibers, carbon fibers and carbon black, pigments and minerals, including silicates, have been used to modify the macroscopic properties of the polymer, such as modulus and toughness. In recent years, a new class of materials have been developed by dispersing layered silicates with polymers at the nanoscale level. These new materials have attracted wide interest because they often exhibit chemical and physical characteristics that are very different from the starting material. In some cases, the silicates and polymers exist as alternating layers of inorganic and organic, as disclosed in Lemmon, J. P.; Wu, J.; Oriakhi, C.; Lerner, M. Electrochim. Acta., 1995, 40, 2245; Vaia, R. A.; Jandt, K. D.; Kramer, E. J.; Giannelis, E. P.; Macromolecules, 1995, 28, 8080; and Tunney, J. J.; Detellier, C. Chem. Mater., 1996, 8, 927. The possibility of improved mechanical, rheological, electrical, and optical properties and the ability to exercise control over existing physical and chemical behavior have led to a large number of studies of these materials, including composites of layer silicate clays with polyethylene oxide (PEO), epoxy resin, polystyrene, and a range of other thermoplastics and elastomers.
Polymer electrolytes exhibit high conductivity only in the absence of a crystalline phase, which impedes the transport of ions, and only at temperatures well above the glass-transition temperature (Tg). A number of methods have been used to prepare totally amorphous polymers of high conductivity, including random copolymers or branched block copolymers. However, the mechanical strength of these polymers is often poor because of their low transition temperatures. Mechanical strength can be maintained by crosslinking of the polymer chains, but this comes at the expense of reduced conductivity. Another approach to increasing conductivity is to incorporate low molecular weight plasticizers into the polymer.
Nanocomposite materials of PEO and phyllosilicates were first suggested by Ruiz-Hitzky and Aranda, Ruiz-Hitzky, E.; Aranda, P. Adv. Mater., 1990, 2, 545, as candidates for polymer electrolytes. Within these materials, the polymer chains are intercalated between the silicate layers. The polymer chains then provide a mobile matrix in which cations are able to move. Nanocomposites of PEO and montmorillonite form a layered aluminosilicate clay. When this composite contains LiBF4, it displays conductivities up to 2 orders of magnitude larger than that of PEO itself at ambient temperatures. However, the addition of lithium salts, which is needed to obtain such conductivity values, is not desirable for two reasons; the first one relates to a more complicated synthetic route and the second relates to the fact that transference numbers are not unity since in this case both cations and anions move.